Electrolyte and method for coating articles with a gold-copper-antimony alloy and article thereof



United States Patent ELECTROLYTE AND METHOD FOR COATING AR- TICLES WITH A GOLD-COPPER-ANTIMONY AL- LOY AND ARTICLE THEREOF Ren Cathrein and Marius Danemark, Geneva, Switzerland, assignors to Sel-Rex Corporation, Nutley, N.J., a corporation of New Jersey No Drawing. Filed June 18, 1965, Ser. No. 465,181

Claims. (Cl. 29-1835) This invention relates to an electroplating process and bath for the production of pink gold deposits consisting essentially of a copper-gold-antimony alloy.

Among the objects of the invention is to provide an improved electroplating bath and process for the production of. bright pink gold deposits.

Acid gold plating baths for the deposition of gold operating at a pH of between 3 and 5 or 6.5 are known. It is also known that bright, hard gold deposits are obtained when base metal ions capable of-codepositing with the gold are added to such baths. See, for example, US. 2,905,601.

One of the most difficult colors to obtain by the acid gold plating processes is a bright gold deposit with a pink coloration. This is difiicult because copper is the main metal ion producing pink coloration and with copper ions the bath is sensitive to changes in concentration, etc., so that uniformly reproducible, copper-gold alloy deposits can be obtained only by careful regulation of the bath components and of the conditions of operation. Also, after a time, a slight haze develops in deposits obtained from such baths, the haze becoming more important as the bath or solution ages.

Among the objects of this invention is to provide a uniformly reproducible bright, hard, copper-gold alloy deposit with a pinkish coloration.

The objects of the invention are attained by providing an acid gold alloy bath having the ingredients listed below and plating a basis metal therewith under conditions of operation which are also listed below:

Range Prefen'ed Gold (added as alkali metal gold cyanide), g./l 1*8 4 Copper (as a copper complex), g./l 10 5 Antimony (added as Sb salt such as the taitrate), 1 5

Additional chelating agent, g Weak acid and salt thereof, g./l 60 pHadjust to 4. 5-6 5. 3 Current density, ampJdmfl- A 52. 0 1. 5 Temperature, C -60 The copper complex is a copper salt of any suitable Chelating or complexing agent having a negative logarithm of stability constant for copper of about 17 or less. Such complexing agents include ethylenediaminetetraacetic acid (EDTA), diethylenetriaminopentaacetic acid, N-hydroxyethylethylenediaminetriacetic acids or the sodium or other alkali metal salts thereof. The excess chelating agent is the same type of compound.

Specular bright deposits are obtained from such baths at thicknesses of 25 microns and over so that the gold layer is thick enough to provide corrosion resistance as well as the pink decoration.

The weak acid is one, as defined in the literature, which has a pK value of over about 3 and such weak acids include the organic acids and phosphoric acid. Where the acid employed is polybasic, the partially neutralized acid salts or mixtures thereof may be employed in place of the acid and salt. Thus KH PO.-,, or corresponding salt of sodium, lithium or ammonium, may be employed in the above bath as the weak acid and salt. Similarly the monopotassium salt of citric acid may be employed as the acid and salt combination.

Although it is preferred to add the antimony as the tartrate, it can also be added as other salts which are soluble in the bath.

Although copper is an essential alloying metal for obtaining a pink coloration in alkali gold cyanide baths, and is sufiicient by itself for the production of pink coloration in such baths, the antimony, as disclosed in this specification, is also essential for the production of pink coloration and specular efiects with the acid gold baths disclosed herein. The alloy deposits obtained from baths as described above consist essentially of to gold, 14 to 24% copper and 0.5 to 1.5% antimony. The alloy has a Knoop hardness of about 230280 kg./mm. with a 25 g. load.

The following example is given to illustrate in detail a preferred bath and method of operating the same. It is to be understood that the specific details given in the example is not to be considered as limiting the scope of the invention.

Example A bath is prepared from the following ingredients:

Brass panels plated to a thickness of 25 microns with the bath at a temperature of 50 C. and at a density of about 1.5 amps/dm. (amperes per square decimeter) have a pink coloration and are similarly bright. The deposit has a hardness of about 240-250 Vickers. The deposits contain about 80% of Au, about 18.9% Cu and 1% of antimony.

In other experiments tetra sodium pyrophosphate and/ or tripolyphosphoric acid, neutralized to a pH of 5.0, was substituted for the potassium dihydrogen phosphate, with substantially the same results. In some of the experiments the acid was as low as 20 g./l., in others as high as g./l. Weak organic acids, partially neutralized to a pH 4.5 to 6.0 produce substantially the same results. When citric acid is employed the brilliance of the deposit is not as great as for the inorganic acids, but the deposit is quite attractive. Tartaric acid, partially neutralized with sodium hydroxide is also satisfactory, although some precipitation of potassium bitartrate occurs when gold is added as the potassium gold cyanide. Baths containing acetic acid-sodium or potassium acetate are satisfactory.

The features and principles underlying the invention described above in connection with specific exemplifications will suggest to those skilled in the art many other modifications thereof. It is accordingly desired that the appended claims shall not be limited to any specific feature or details thereof.

We claim:

1. Process for electrodepositing bright, hard gold-copper alloys having a pink coloration which comprises electrolyzing an aqueous solution comprising the following ingredients:

G./l. Gold (added as alkali metal gold cyanide) 18 Copper (added as a copper complex) 2-10 Antimony (added as a salt) 0.5-l0 Chelating agent (in addition to that complexed with the copper) 5-60 Weak acid and salt thereof, and in proportion to provide a pH of 4.5-6,

maintaining said bath at a temperature of 40 to 60 C. and regulating the current density to a value of 0.5 to 2.0 amps/dmfi.

2. The process as set forth in claim 1 wherein the bath contains the following ingredients:

G./l. Gold (added as alkali metal gold cyanide) About 4-8 Copper salt of Na EDTA (calc. as Cu) About 5 Antimony tartrate (calc. at Sb) About 1.5 Na EDTA About 20 KI'I2PO4 About said bath being operated at a pH of about 5.3, a temperature of about 50 C. and a current density of about amps/dmF.

3. An aqueous bath for plating hard, specularly bright 4. An aqueous bath for plating hard, specularly bright copper gold alloy consisting essentially of the following:

Y i G./l. Gold (added as potassium gold cyanide) About 4 Copper salt of Na EDTA (calc. as Cu) About 5 Antimony tartrate (calc. as Sb) About 1.5 Na EDTA About 20 KH2PO4 About adjusted to pH to about 5.3.

5. As an article of manufacture a substrate having a conducting surface which is electroplated to a thickness of at least microns with a coating consisting essentially of an alloy of to 85% gold, 14 to 24% copper and 0.5 to 1.0% of antimony.

References Cited UNITED STATES PATENTS 2,724,687 11/1955 Speter et a] 204-43 2,905,601 9/1959 Rinker et a1. 204--44 XR 2,967,135 1/1961 Ostrow et al 204--46 XR 3,020,217 2/1962 Rinker 20446 XR OTHER REFERENCES Thews, Edmund R.: The Production of Colored Gold Finishes, Metal Finishing, pp. 85, September 1951.

Pritchard, Alfred K., Jr.: Deposition of Pink Gold, Metal Industry, vol. 31, No. 12, p. 408, 1933.

HOWARD S. WILLIAMS, Primary Examiner.

G. KAPLAN, Assistant Examiner. 

1. A PROCESS FOR ELECTRODEPOSITING BRIGHT, HARD GOLD-COPPER ALLOYS HAVING A PINK COLORATION WHICH COMPRISES ELECTROLYZING AN AQUEOUS SOLUTION COMPRISING THE FOLLOWING INGREDIENTS:
 5. AS AN ARTICLE OF MANUFACTURE A SUBSTRATE HAVING A CONDUCTING SURFACE WHICH IS ELECTROPLATED TO A THICKNESS OF AT LEAST 25 MICRONS WITH A COATING CONSISTING ESSENTIALLY OF AN ALLOY OF 75 TO 85% GOLD, 14 TO 24% COPPER AND 0.5 TO 1.0% OF ANTIMONY. 